\begin{tikzpicture}
    \pgfmathsetmacro{\factor}{0.5}
    \pgfmathsetmacro{\a}{4 * \factor}
    \pgfmathsetmacro{\b}{3 * \factor}
    \pgfmathsetmacro{\c}{5 * \factor}

    \tkzDefPoints{0/0/A, \a+\b/0/B, \a+\b/\a+\b/C, 0/\a+\b/D}
    \tkzDefPoints{\a/0/E, \a+\b/\a/F, \a/\a+\b/G, 0/\a/H}
    \tkzInterLL(E,G)(F,H)  \tkzGetPoint{O}

    \tkzDrawPolygon(A,B,C,D)
    \tkzFillPolygon[gray!20](D,H,O,G)
    \tkzFillPolygon[gray!20](E,B,F,O)
    \tkzDrawSegments(E,G  F,H  D,O  B,O)

    \tkzLabelSegment[above](D,G){$a$}
    \tkzLabelSegment[above](G,C){$b$}
    \tkzLabelSegment[left](A,H){$a$}
    \tkzLabelSegment[left](H,D){$b$}

    \tkzLabelSegment(A,O){I}
    \tkzLabelSegment(O,C){II}
\end{tikzpicture}

